Known systems for performing radiographic inspection of an object include systems which utilize X-ray film. Such inspection systems generally produce high resolution images of the object being inspected, but present a disadvantage in that such systems perform radiographic reproduction over a limited dynamic range. The high resolution of the X-ray film results from the fact that individual silver bromide crystals, which make up the emulsion layer of the X-ray film and which are activated by incident X-rays to create a "picture," are of relatively small size. The dynamic range, on the other hand, is determined by the number of silver bromide crystals per unit area in the emulsion layer that can be exposed to the X-ray radiation. The dynamic range of a given film is limited by the thickness of the silver bromide crystal emulsion that, when developed, will not allow detectable light to be transmitted. The use of higher density film having high dynamic range necessitates the use of a very intense light source for reading the X-ray piCture. This requirement for a very intense light for reading the X-ray film presents a significant disadvantage as to the use of such film. Also, the use of X-ray film for radiographic inspection does not permit compensation for the effects of radiation scattered by the object being inspected.
To increase the dynamic range for radiographic inspection systems and to permit compensation for radiation scatter, radiation detectors that produce a linear electronic signal response, when exposed to a wide range of X-ray intensities, in conjunction with electronic processing of the signals with wide range linear amplifiers are used.
An example of a known radiographic inspection system, which utilizes electronic detectors, is a "linear flying spot scanner" of the type commonly used for inspecting luggage at airports. This linear flying spot scanner includes a radiation source, a stationary shield, a rotating beam-chopper wheel and electronic detectors. The radiation source transmits a beam of radiation along a path toward a selected location at which an object is positioned. The stationary shield, rotating beam-chopper wheel and the electronic detectors are disposed in the radiation path, with the stationary slit and beam-chopper wheel positioned between the radiation source and the electronic detectors.
The stationary shield and beam-chopper wheel are aligned to select a portion of the radiation beam from the source. This portion of the radiation beam is transmitted along the radiation path to a single portion of the selected location at which the object is positioned and to a corresponding selected portion or area of the object.
Radiation transmitted to the selected area of the object interacts with the selected area. A portion of the radiation interactive with the area of the object is compton scattered in all directions. Another portion is absorbed by the photoelectric interaction in the object and re-emitted in all directions as fluorescent radiation, while yet another portion passes through the object. X-ray sensitive electronic detectors, disposed between the radiation source and the object, generate signals in response to the fluorescent and compton scattered radiation which is scattered in the backward direction by the selected area of the object, and through-radiation detectors, positioned such that the object is between the radiation source and the through-radiation detectors, generate response signals for radiation passing through the selected area of the object.
Response signals generated by the electronic detectors are processed to obtain a data representation of the selected area of the object. The signal processing is performed by a data processor. An image of the object area is obtained using the processed signal data, and using a visual display means which is responsive to the processed data, the image of the selected area of the object is displayed.
As described above, the stationary shield and beam chopper wheel select a single line portion of the selected location and thereby allow radiation to be transmitted to just a corresponding selected area of the object. By limiting the portion of the object to which radiation is transmitted, the exposure of the object to radiation is limited. The number of detectors required to generate response signals is also minimized by limiting the radiation transmitted to the object and, thus, just a few large area detectors are utilized. The resolution of such a system is determined by the size of the slits in the stationary shield and the rotating chopper wheel.
However, because the linear flying spot scanner only operates to select a single line portion of the selected location and a single corresponding object area, the necessity exists to move the object through the selected inspection location in order to radiographically inspect area of the object other than the single line selected area. Such movement places successive line areas of the object in the selected location to which radiation is transmitted and thereby permits a radiographic image of each line area of the object to be obtained.
The requirement of moving the object relative to the linear flying spot scanner is a major disadvantage in that, in order to move the object, specialized moving apparatus is needed. Furthermore, space must be provided with the linear flying spot scanner to accommodate the moving apparatus.
Alternatively, certain known radiographic inspection systems utilize a collimated linear array of electronic detectors. The electronic detectors are moved relative to the object and radiation source to successively generate response signals for the successive sections or areas of the object. The radiation source of such a system transmits radiation toward the entire object being inspected, permitting the detector array, as moved, to generate response signals for each section of the object. The moving detector system requires a large number of electronic detectors to inspect the object to ensure high resolution. The major disadvantages of such a system are the need to move the detectors relative to the object and the large number of detectors and associated electronics. The detectors must be balanced for uniform response characteristics.